Iron and steel products become coated with an oxide scale during their process of manufacture; the presence of this scale is objectionable when the material is processed further. For example, the oxide scale must be removed and a clean surface provided if satisfactory results are to be obtained from hot-rolled sheet or strip in any operation involving deformation of the product. If sheets are manufactured for drawing applications, removal of the oxide scale is essential, as its presence on the steel surface tends to shorten die life, cause irregular drawing conditions, and destroy the surface smoothness of the finished product. If the sheet or strip is to be used in further processing involving coating, oxide removal is necessary to permit proper alloying or adherence of metallic coatings and satisfactory adherence when a non-metallic coating or paint is used. In the production of cold-reduced steel sheet and strip, it is necessary that the oxide resulting during hot rolling of the slab to breakdown form be removed completely before cold reduction to prevent lack of uniformity and eliminate surface irregularities.
Pickling is the chemical process used to remove the oxide by the action of water solutions of inorganic acids, usually either sulfuric acid (H.sub.2 SO.sub.4) or hydrochloric acid (HCl). When steel or iron materials are immersed in solutions of these acids a portion of the oxide scale is dissolved and, to some extent, the acid penetrates cracks in the scale and a portion of the scale is undercut and lifted off. Unless an inhibitor is added to the pickling solution excessive base metal dissolution may occur in a batch operation or in a continuous operation during a line stop. Further, large quantities of spent pickle liquor result from these operations and, though recovery methods have been developed, they involve appreciable capital investment and produce by-products such as hydrated ferrous sulfate or finely divided iron oxide mixtures that, themselves, present disposal problems. The matter is complicated by pollution abatement legislation that limits the applicability of neutralization and dumping of sludge that has been largely employed heretofore.
We are not aware of any references in the literature to the use of fluoboric acid for chemical pickling (absence of electric current) hot rolled steel.
The reader may be interested in reviewing DeBruyne U.S. Pat. No. 3,654,099, which describes the use of HBF.sub.4 in an aqueous acid bath for cathodic treatment of stainless steel to activate it prior to electroplating. D'Aquila et al, in U.S. Pat. No. 2,920,023 employs a fluoborate salt in a molten bath for electrolytic cleaning of ferrous metal cleaning.
For general background, the following U.S. Patents may also be of interest: U.S. Pat. Nos. 3,694,334, 3,383,324, 3,280,038, 3,230,172, 3,304,246, 3,096,261, 2,773,023, 2,760,927, 3,627,654, 3,660,253 and 3,293,159.
The use of an anion permselective membrane to remove iron values from spent fluoboric acid has also not been suggested prior to our invention, so far as we know.